Growth and reproduction of the green sea urchin Strongylocentrotus droebachiensis across a depth gradient

Abstract

During periods of high water temperature along the Atlantic coast of Nova Scotia, the green sea urchin, Strongylocentrotus droebachiensis, is susceptible to infection by Paramoeba invadens, a pathogenic amoeba that can eliminate urchins from the shallow rocky subtidal zone along hundreds of kilometres of shoreline. Repopulation of disease affected areas by S. droebachiensis was monitored at a wave-exposed site (Chebucto Head) following a mass mortality in September 1999 at 3 depth strata ( 12, 16 and 24 m) and later at a grazing front. The primary means of repopulation was migration of adult urchins from deeper water while recruitment played a secondary role. Low temperatures below 20 m depth provided urchins with a thermal refuge from disease and a bedrock ramp at the study site provided a direct migration route to shallow water. Far-field surveys revealed an extensive deep-water population with an average density of 75 urchins m-2 on hard substratum. At 16 m, urchin density stabilized within 6 mo of the dieoff while repopulation at 12 m took closer to 2 y. Within 2.5 y, a grazing front of large urchins ( 40-60 mm) had formed along the lower margin of a kelp bed at 10 m depth, with densities up to 284 urchins m-2.

As urchins migrated from the food-limited persistent barrens at 24 m, they responded rapidly to increased food quality and quantity in the shallows through increased growth and reproduction. Measures of annual increments of skeletal elements (rotules) from urchins across the depth gradient indicated that the fastest growing individuals from the source population formed the grazing front. Urchins in the front reached a larger asymptotic size and produced more gonad than urchins lower on the ramp. The annual cycle in gonad index showed a pronounced spring spawning period across all depths and a secondary fall spawning. The presence of mature, fertilizable ova and short response time to spawning induction in both spring and fall supported the occurrence of two spawning periods.

Estimates of egg production across a depth gradient indicated that the urchin population in deep water spawned more eggs per unit area than either depth stratum in shallow water (12 and 16 m) during the first 2 y of repopulation. A higher urchin density at 24 m compensated for a smaller test diameter and lower gonad production, until a grazing front formed. Egg production in the spring increased over time at shallower depths but remained low in the fall . As a function of fecundity and density, fertilization rates and hence zygote production followed the same patterns as egg production. When the site was divided into two zones of equal area (1000 m2), persistent barrens (24 m) and transitional barrens (16, 12 m and the front), urchins in the persistent barrens produced a greater proportion of the total number of eggs spawned ( or zygotes produced) at the site, until density in the transitional barrens increased and a front formed after 2 y.